1. Field of the Invention
The present invention relates to a bonding material in a semiconductor module and a method for manufacturing the bonding material.
2. Background Art
In a non-insulated semiconductor apparatus which is one of power semiconductor apparatuses used in an inverter or the like, a member which fixes a semiconductor device also serves as an electrode of the semiconductor apparatus. For example, in a semiconductor apparatus in which a power transistor is mounted on a fixed member using a Sn—Pb soldering material, the fixed member (base material) serves as a collector electrode of the power transistor. When the semiconductor apparatus is in operation, a current of several amperes or more flows through the collector electrode portion, and the transistor chip generates heat. To avoid destabilization of characteristics and reduction in lifetime caused by the heat generation, the heat dissipation and long-term reliability (heat resistance) of a soldering portion need to be ensured. A material with high heat dissipation is required to ensure the heat resistance and heat dissipation of the soldering portion.
It is necessary also for an insulated semiconductor apparatus to efficiently dissipate heat generated when the semiconductor apparatus is in operation out of the semiconductor apparatus and ensure the connection reliability of a soldering portion in order to safely and stably operate a semiconductor device.
As a connecting material with high heat dissipation and reliability, there is known a conductive adhesive using a conductive composition including a particulate silver compound (e.g., JP Patent Publication (Kokai) No. 2003-309352 A (2003), hereinafter referred to as Patent Document 1). However, since a method using such a conductive adhesive uses a binder as a bonding mechanism at an interface, it is inferior in heat dissipation and bonding reliability to a method for achieving metallic bonding at an interface.
It is known that if the particle diameter of particles of a metal decreases to 100 nm or less, and the number of constituent atoms decreases, the ratio of surface area to volume of the particles rapidly increases, and the melting point and sintering temperature become much lower than those of the metal in bulk state. There is known bonding utilizing the property of sintering at a low temperature, i.e., bonding by using metal particles whose surfaces are coated with an organic substance and which have an average particle diameter of 100 nm or less, decomposing the organic substance by heating, and sintering the metal particles (e.g., JP Patent Publication (Kokai) No. 2004-107728 A (2004), hereinafter referred to as Patent Document 2). After bonding by the bonding method, metal particles have changed into bulk metal, and metallic bonding is achieved at a bonding interface. The method thus has very high heat resistance, reliability, and heat dissipation. Although solder is under pressure to be lead-free, there is as yet no material to replace high-temperature solder. Since use of hierarchical soldering is indispensable for mounting, there is a demand for the advent of a material to replace high-temperature solder. Therefore, the bonding technique is expected as a material to replace high-temperature solder.
Note that JP Patent Publication (Kokai) No. 2006-41008 A (2006) (hereinafter referred to as Patent Document 3) discloses that connection reliability at an interface and closeness of a bonding layer are ensured by producing silver nanoparticles during heating at the time of bonding to fill interstices in conductive resin on the spot, which makes it possible to improve conductivity and electrical connection reliability.
In the bonding method using metal particles having an average particle diameter of 100 nm or less disclosed in, e.g., Patent Document 2, since bonding is achieved by metallic bonding at a bonding interface, as described above, the bonding method has high heat resistance, reliability, and high heat dissipation. However, since very fine metal particles having an average particle diameter of 100 nm or less tend to cohere, it is necessary to form a protective film made of an organic substance in order to stabilize such metal particles. Although the protective film of the organic substance needs to be removed at the time of bonding, it is difficult to completely remove the protective film by heating at a low temperature, and sufficient bonding strength is hard to achieve. If a protective film made of an organic substance for metal particles is molecularly designed to decompose at a low temperature, metal particles produced at a room temperature of 20 to 30° C. immediately cohere, and thus it is difficult to produce metal particles which can sinter at a low temperature. In addition, since production of metal particles having an average particle diameter of 100 nm or less involves troublesome work such as removal of impurities after the production, reducing the cost of a bonding material is difficult. As described above, a bonding method using metal particles having an average particle diameter of 100 nm or less has practical problems remaining to be solved such as production of metal particles, removal of impurities from and storage of metal particles after production, and handling of metal particles.
In a bonding method disclosed in Patent Document 3, 5 to 50 parts by mass of tertiary fatty acid silver salt per 100 parts by mass of a conductive composition is mixed in the conductive composition, silver nanoparticles are produced on the spot at the time of bonding, and interstices in resin is filled after thermal hardening, thereby improving conductivity and the like. However, the volume of tertiary fatty acid silver salt, which is a precursor of a silver nanoparticle, shrinks, the content of silver in the tertiary fatty acid silver salt is not high, and the volatilization temperature of an organic substance generated when the tertiary fatty acid silver salt decomposes is not low. Accordingly, it is difficult to use more than 50 parts by mass of the silver nanoparticle precursor per 100 parts by mass of the conductive composition in a bonding material. Additionally, since a heating time of 10 minutes or more is required, shortening of process time is difficult. Moreover, since adhesion is achieved mainly by using a binder at an interface, as in Patent Document 1, it is difficult to acquire a close sintered silver layer and achieve metallic bonding at an interface after bonding.